Import for power to work with pyobjc 3

From what I've looked at, it seems that there is an issue with 'super' in python that prevented this from working with pyobjc 3. I filed a bug report [here](https://bitbucket.org/ronaldoussoren/pyobjc/issue/110/subclass-of-nsobject-super-has-no), and this is what they suggested to do. I tested it with pyobjc version 2.5.1 and 3.0.4, and they both seem to work on my mac, version 10.9.5.